1894 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



bicarbonate to serve as vehicle for carrying carbon dioxide into the cell. 

 The anatomy of some aquatic plants, on the other hand, is such that carbon 

 dioxide has to diffuse through a liquid layer up to 40 ^ thick to penetrate 

 from the surface of the plant to some of the chloroplasts. Such plants 

 would starve if they had to rely entirely on the supply of CO2 molecules by 

 diffusion from a medium containing only about 1 X 10 ~^ mole/1. CO2. 

 In fact, Steemann-Nielsen found the photosynthesis of Myriophyllum 

 spicatum and Fontinalis antipyretica to reach saturation, in acid solution 

 (containing free CO2 only), only when [CO2] exceeded 50 X 10 ~^ mole/1. 

 (The removal of thick terminal buds reduced, in the case of Fontinalis, the 

 concentration needed for saturation, to [CO2] = 20 X 10 ~^ mole/1.) In 

 such plants, the rate of photosynthesis at [CO2] = 1 X 10 ~^ mole/1, (water 

 equilibrated with free air) would be insignificant, unless the medium con- 

 tained also bicarbonate, and the cell walls were permeable to it. As an 

 illustration of these relationships, fig. 37D.3 compares once again the carbon 

 dioxide curves (P = /[CO2]) in a medium containing CO2 molecules only, 

 of a terrestrial plant (Triticum), a terrestrial alga (Hormidium), and an 

 aquatic plant (Myriophyllum) . 



Steemann-Nielsen pointed out that, even in the case of Myriophyllum 

 (not to speak of Hormidium), the pairs of curves in fig. 37D.3 which corre- 

 spond to different light intensities start with the same slope ("Black- 

 man type," cf. fig. 26.2). He suggested that carbon dioxide curves of the 

 "Bose type" (fig. 26.3) — such as had been found by Harder for Fontinalis 

 (fig. 27.2A) — are always caused by insufficient rate of supply. (He could 

 obtain such curves also with Myriophyllum, by using stagnant water.) Ac- 

 cording to chapter 27 (section A7b), diffusion resistance cannot produce 

 curves of the "Bose type" ; Steemann-Nielsen suggested, however, that such 

 curves could be explained by convection, if it is assumed that the rate of the 

 latter increases with the intensity of illumination; this would raise the 

 "roof" imposed on the carbon dioxide curves of photosynthesis by CO2 

 supply limitation {cf. fig. 26.6). 



It was shown in chapter 27, that CO2 curves of the "Bose type" must result also 

 when the factor detei'mining the shape of the curves is the carboxylation equihbrium, 

 A + CO2 ;=^ ACO2. We expect this relationship to become revealed when all CO2 con- 

 centration gradients are practically eliminated, and all "supply roofs" thus hfted. It is 

 unlikely that these conditions could have been realized in Harder's measurements; his 

 CO2 curves rise much too slowly for such an interpretation. 



Steemann-Nielsen (1952) criticized also the carbon dioxide curves Smith had given 

 for Cabomba caroliniana (fig. 27.4), because of the use of "unphysiological," pure potas- 

 sium carbonate buffers, and the curves Emerson and Green gave for Chlorella pyrenoidosa 

 (fig. 27. 2C) because their method (p. 897) did not take into account the possible contribu- 

 tion to photosynthesis, in a closed volume of gas, of intercellular carbonate reserves 

 (which Steemann-Nielsen believes all cells must contain, cf. p. 196). 



